CN107108946B

CN107108946B - CNF porous solid material with anionic surfactant

Info

Publication number: CN107108946B
Application number: CN201580059005.4A
Authority: CN
Inventors: E.约翰逊; N.张塞尔温; L-E.维格伯格
Original assignee: Cellutech AB
Current assignee: Cellutech AB
Priority date: 2014-10-30
Filing date: 2015-10-29
Publication date: 2021-03-09
Anticipated expiration: 2035-10-29
Also published as: US20170313838A1; CN107108946A; WO2016068787A1; JP6787886B2; PL3212845T3; CA2963633A1; US10907020B2; HK1243111A1; EP3212845A1; EP3212845B1; JP2017535641A

Abstract

The present invention relates to porous solid materials comprising Cellulose Nanofibers (CNF) and anionic surfactants, to a process for preparing the materials, and to their use.

Description

CNF porous solid material with anionic surfactant

Technical Field

Background

In our daily lives, macroporous and microporous materials made primarily of petroleum-based polymers are used in a variety of forms and compositions. Examples of these are building and aircraft insulation and polymer foams for packaging. Foams for such use must be stable, lightweight, and easy to manufacture.

Due to the increased awareness of the need to use renewable materials, there is a strong need to use polymers derived from renewable resources instead of petroleum-based polymers. Cellulose, being the most abundant renewable natural polymer on earth, has a crystal structure and is suitable for large-scale industrial preparation methods, and therefore has unique potential. The long nanofibrils in the plant are filled with cellulose chains with repeating units of β - (1-4) -D-glucopyranose, with cross-sectional dimensions of 5-30nm, depending on the plant origin. The parallel structure of cellulose chains is bonded together by hydrogen bonds and forms a sheet, thereby constituting a crystal structure having a young's modulus of about 130 GPa.

Given their excellent mechanical properties, these nanofibrils are very interesting as potential building blocks for new nanomaterials and as alternative materials for petroleum based structures. Cellulose Nanofibrils (CNF) can be released from wood pulp by mechanical disintegration, usually first by enzymatic disintegration (Henriksson et al, European Polymer Journal, 2007,43(8) 3434-;

et al, Langmuir, 2008, 24784-. The terminology in the literature is not explicit and nanofibrils have been referred to as e.g. microfibrillated cellulose (MFC), nanofibrillated cellulose (NFC), and Cellulose Nanofibrils (CNF) as used herein.

The use of colloidal particles to stabilize high energy interfaces in so-called Pickering emulsions has been known for over a century. This concept has not until recently been applied to the preparation of ultra stable wet foams and to the preservation of these structures in the dry state to maintain porous materials. Particles may adhere to the gas-liquid interface when partially lyophobic or hydrophobic. This is because it energetically favors particle adhesion to the gas-liquid interface and replaces part of the energetic solid-liquid region by a low energetic solid-gas region. Preferably, the particles adhere to the interface with a contact angle of about 90 °. This is ultimately determined by the balance between gas-liquid, gas-solid and solid-liquid interfacial tensions. Since particles have high adsorption energy, they tend to adsorb more strongly at the interface than surfactants. Thus, particle-stabilized foams (particle-stabilized foams) exhibit superior stability compared to surfactant-based systems. Coalescence (coalescence) is hindered by the steric repulsion of the adsorbed particles and the formation of a layer of particles at the interface effectively prevents the contraction and expansion of the bubbles, thereby minimizing oswald ripening and forming a long lasting stable foam.

WO2007/068127a1 discloses how to prepare particle-stabilised foams in which the initial lyophilic particles are lyophobic in situ by adsorption of amphiphilic molecules on the surface of the particles. These foams can be dried to preserve the porous structure (Studart et al, j.am. center. soc., 2006, 89(6) 1771-. Foams prepared with particles and non-adsorbing amphiphiles, conventional non-ionic surfactants or surfactants having the same charge sign as the particles are unstable and collapse within seconds to minutes (Gonzenbach et al, Langmuir 2006, 22, 10983-.

WO2014/011112a1 discloses the preparation of hydrophobic wet foams from anionic CNFs which are lyophobic by adsorption of cationic hydrophobic amines, such as n-octylamine. The foam is dried to maintain a Porous structure (Cervin, Porous Materials from Nano fibrous Cellulose, Royal Institute of Technology, 2012; Cervin et al Lightweight and Strong fibrous Materials from amino foam Stabilized by Nanofibrillated Cellulose, Biomacromolecules,2013,14, 503-311; WO2014/011112A 1).

Foams formed from a web containing CNF and pulp (WO 2013/160553) and a Thin foam coating of CNF on the web have been prepared In Technical Research Centre of Finland (VTT) (Kinnunen et al, Thin coatings for paper by foam coating, In paper Con2013, 27April-1May, Atlanta, GA: TAPPI).

Most processes for producing solid porous materials comprising CNF involve supercritical drying or freeze drying of wet CNF gels. However, it is difficult to prepare a large block foam without cracks using this method.

CNF is very interesting for the preparation of highly porous renewable materials, and a lot of work has been done in this respect. However, there is a need for an improved process wherein wet CNF foams can be dried while maintaining the porous structure in the dry state.

Disclosure of Invention

It is an object of the present invention to provide a porous solid material derived from renewable materials. It is another object of the present invention to provide renewable materials having a high degree of porosity, a good pore size distribution and good mechanical properties.

The present invention relates to a porous solid material comprising Cellulose Nanofibers (CNF) and a surfactant, wherein:

a) the surfactant is an anionic surfactant;

b) said material having a density of less than 500kg/m³(ii) a And is

c) At least 50% of the pores of the material have a diameter of at least 10 μm.

The invention further relates to a method for preparing a porous solid material comprising:

a) providing a dispersion of Cellulose Nanofibers (CNF) in an aqueous solvent,

b) adding an anionic surfactant to the dispersion in (a) to obtain a mixture,

c) preparing a foam of the mixture obtained in b), wherein the wet foam has a density of at most 98% of the mixture prepared in step (b), and

d) drying the foam obtained in c) to obtain a porous solid material.

The invention also relates to a porous solid material obtainable by the process according to the invention, the use of a porous solid material comprising Cellulose Nanofibres (CNF) and an anionic surfactant for insulation, packaging or absorption, and a product comprising a porous solid material comprising Cellulose Nanofibres (CNF) and an anionic surfactant.

The porous solid material according to the invention has a maintained porous structure and shows good mechanical properties.

Brief Description of Drawings

Figure 1 shows the wet foam stability of CNF foams prepared with different surfactants.

Fig. 2 shows the complex elastic modulus of CNF with or without added surfactant.

Fig. 3 shows the air content (. diamond-solid.) in the wet foam and the density (. diamond-diamond.) of the porous solid material as a function of the concentration of the CNF dispersion, wherein the SDS loading was kept constant at 0.1ml SDS solution (20g/L) in 30g of CNF dispersion.

FIG. 4 shows the air content (. diamond-solid.) in the wet foam and the density (. diamond-diamond.) of the dried porous solid material as a function of the concentration of the CNF dispersion, wherein the SDS loading is 20mg SDS/g CNF.

FIG. 5 shows SEM images of porous solid materials prepared from 30g TEMPO-oxidized anionic CNF (0.6 wt%) and 0.1ml anionic SDS (25 g/L).

FIG. 6 shows an SEM image of a porous solid material prepared from 300g TEMPO-oxidized anionic CNF (0.5 wt%) and 1.0ml anionic SDS (25 g/L).

Detailed Description

Unless otherwise indicated, all words and abbreviations used in this application shall be interpreted as having the meaning commonly given to them in the relevant art. For clarity, certain terms are specifically defined below.

In this specification, the term "foam" is used to refer to a chamber in which a gas is dispersed in a solid or liquid medium, wherein the bubbles are separated from each other by a thin film of the liquid or solid medium to form the gas.

In this specification, the term "porous solid material" is used to refer to a solid material having a combination of cells with solid edges or faces that are clustered together. Fig. 5 and 6 show typical porous solid materials.

In the context of the present application, the term "diameter" refers to the largest internal dimension of the cell.

The term "CNF" as used herein is used to refer to cellulose nanofibers released from wood pulp or other sources, for example selected from plants, tunicates (tunicates) and bacteria, by mechanical disintegration, typically first by chemical pretreatment, such as by oxidation with 2,2,6, 6-tetramethylpiperidine-1-oxide (TEMPO), to give TEMPO-oxidized CNF, or by carboxymethylation to give carboxymethylated CNF; or by enzyme-treatment, such as by endoglucanase, to obtain the enzyme-modified CNF. CNFs typically have a minimum dimension in the range of 2-100nm, while the length can be a few microns, e.g. up to 10 μm, so the aspect ratio (length to diameter ratio) of CNFs is very large. One advantage of using a CNF derived from wood pulp is that wood-based cellulose is very abundant and there is an efficient infrastructure for handling and processing wood pulp and fibers.

In a first aspect, the present invention relates to a porous solid material comprising Cellulose Nanofibers (CNF) and a surfactant, wherein:

a) the surfactant is an anionic surfactant;

b) said material having a density of less than 500kg/m³(ii) a And

In a second aspect, the present invention relates to a method of preparing a porous solid material comprising:

a) providing a dispersion of Cellulose Nanofibers (CNF) in an aqueous solvent,

b) adding an anionic surfactant to the dispersion in (a) to obtain a mixture,

c) preparing a wet foam of the mixture obtained in b), wherein the wet foam has a density of less than 98% of the mixture prepared in step (b), and

d) drying the wet foam obtained in c) to obtain a porous solid material.

In a third aspect, the present invention comprises a porous solid material obtainable by the method according to the second aspect of the present invention.

One advantage of the porous solid material according to the invention is that it is prepared from renewable materials. The solid porous material according to the present invention may comprise at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt% CNF, calculated on the total weight of the porous solid material. The solid porous material according to the present invention may comprise up to (and including) 99.8 wt% CNF, up to (and including) 99.5 wt% CNF, up to (and including) 99 wt%, up to (and including) 98 wt%, up to (and including) 97 wt%, up to (and including) 96 wt%, up to (and including) 95 wt%, up to (and including) 90 wt%, up to (and including) 80 wt%, or up to (and including) 70 wt% CNF, calculated on the total weight of the porous solid material.

The concentration of CNF used in the method of the invention can be varied; CNF quality, such as pretreatment, charge and homogenization of nanofibers; selection and amount of surfactant; an additive; mixing energy; and the amount of gas introduced during the foaming step to adjust the properties of the porous solid material.

The porous solid material according to the invention has a density of less than 500kg/m³. In a preferred embodiment, the porous solid material according to the invention may have a density of less than 300kg/m³Less than 200kg/m³Less than 100kg/m³Or less than 50kg/m³The density of (c). The porous solid material may have a density of at least 0.5kg/m³Or at least 1.0kg/m³。

The viscosity of the CNF dispersion in step (a) of the process will increase significantly with increasing CNF concentration. This affects the amount of gas that can be introduced with a given foaming process and the degree of stability of the wet foam.

In the process according to the second aspect of the invention, the CNF concentration in the dispersion in step (a) may be at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt%, or at least 0.5 wt%, calculated on the total weight of the dispersion.

Dispersions of at least 1 wt% CNF, calculated on the total weight of the dispersion, may also be used in the process according to the invention. Higher concentrations of CNF (e.g., above 1 wt%) reduce the time required to dry the foam. By varying the concentration of CNF, the properties of the porous solid material can be adjusted. Since the viscosity of the CNF dispersion increases significantly when the CNF concentration increases, the upper limit of the CNF concentration depends on the available foaming settings, e.g. the capacity of the mixer. Typically, the concentration of CNF in the dispersion of step (a) may be up to (and including) 20 wt%, or up to (and including) 15 wt% CNF, based on the total weight of the dispersion.

The aqueous solvent used for the preparation of the CNF dispersion in the process according to the invention may be water, or a mixture of water and an organic solvent such as ethanol. Such a mixture of water and organic solvent may have a water content of at least 80%, at least 85%, at least 90%, or at least 95%, based on the total weight of the aqueous solvent.

The porous solid material and the CNF used in the preparation method thereof according to the present invention may be cellulose nanofibers selected from the group consisting of: enzyme-modified CNF, TEMPO-CNF and carboxymethylated CNF.

The cellulose nanofibers of the present invention can be an anionic surfactant. The charge density of anionic cellulose nanofibers will depend on the degree of modification obtained by chemical pretreatment of cellulose before mechanical decomposition into CNFs. The anionic Cellulose Nanofibers (CNF) used in the present invention may have the following charge density: 0 to 2000 mu eq/g, 25 to 2000 mu eq/g, 200 to 2000 mu eq/g, 0 to 1500 mu eq/g, 25 to 1500 mu eq/g, 250 to 1500 mu eq/g, 500 to 1500 mu eq/g or 750 to 1500 mu eq/g, calculated on the dry weight of the CNF. Charge density can be determined by conductometry, as described by Katz et al, Svensk Papperstidning 1984, R87, or by polyelectrolyte titration, as described

L, et al Nord.Pulp Pap.Res.J.1989, 4, 71-76.

With the method according to the invention, a porous solid material comprising Cellulose Nanofibres (CNF) can be formed by: the dispersion of CNF and at least one anionic surfactant is foamed without using other kinds of surfactants. The benefit of anionic surfactants is that most of these surfactants are non-toxic, having an LD50 comparable to sodium chloride.

The porous solid material according to the present invention comprises one or more anionic surfactants. In a particular embodiment of the invention the porous solid material comprises only surfactants selected from anionic surfactants. Examples of suitable anionic surfactants are anionic surfactants selected from the group consisting of: sodium Dodecyl Sulfate (SDS), Sodium Lauryl Ether Sulfate (SLES), sodium oleate, and potassium oleate, or a combination thereof. In particular, the anionic surfactant used in the porous solid material according to the present invention may be Sodium Dodecyl Sulfate (SDS).

The present invention therefore relates to a porous solid material comprising CNF and one or more anions and a process for its manufactureA surfactant, wherein the porous solid material has a density of less than 500kg/m³(ii) a And at least 50% of the pores of the material have a diameter of at least 10 μm.

The porous solid material according to the invention may also comprise a combination of anionic and nonionic surfactants, for example in detergents and wash liquors.

Furthermore, the porous solid material according to the present invention may be provided having a thickness of at least 0.05mm, at least 0.1mm, at least 0.2mm, at least 0.5mm, at least 1mm, at least 2mm, at least 5mm or at least 10 mm. The porous solid material may be provided in a thickness of up to (and including) 100cm, 50cm, or up to (and including) 20 cm.

Unlike cationic surfactants, anionic surfactants do not adsorb to the anionic CNF and therefore do not render the CNF surface active. Thus, it cannot be expected that with the process according to the invention aqueous dispersions of anionic CNF and anionic surfactant can be formed into wet foams which are so stable that they can be dried into porous solid materials with retained porous structure without the use of freeze drying, supercritical drying or cationic surfactants. Surprisingly, wet foams prepared from CNF dispersions of at least 0.2 wt% CNF in combination with an anionic surfactant such as SDS can be dried to have a retained porous structure without using freeze drying or supercritical drying to give porous solid materials according to the present invention.

One advantage of using anionic surfactants is that very small amounts of such surfactants can be used to prepare the porous solid material according to the present invention. The porous solid material according to the present invention may comprise less than 5 wt%, less than 4 wt%, less than 3 wt% or less than 2 wt% of anionic surfactant, based on the total weight of the porous solid material. The porous solid material according to the present invention may comprise at least 0.2 wt%, or at least 0.5 wt% anionic surfactant.

After addition of the surfactant in step (a) of the process according to the invention, the pH of the resulting mixture may be adjusted to a pH above 4.5, for example above pH 5, above pH 6, above pH 7, above pH8, or above pH 9. The pH of the mixture in step (a) may be no more than pH 13, no more than pH 12, or no more than pH 11.

The density of the wet foam obtained as an intermediate in step (c) of the process according to the invention is less than 98%, less than 90%, less than 80%, less than 70%, less than 60%, less than 50%, less than 40%, less than 30%, less than 20% of the mixture prepared in (b). It is possible that the amount of gas introduced in the foaming step (b) will have an effect on the lowest possible density of the resulting wet foam. The amount of gas that may be introduced will depend on the method of introducing the gas, the viscosity of the CNF dispersion, and the type and amount of surfactant. The density of the wet foam obtained as an intermediate in step (c) of the process according to the invention is at least 1%, at least 5%, at least 10%, at least 15%, at least 20%, or at least 25% of the mixture prepared in (b).

The foaming in step c) of the process may be carried out by introducing a gas into the mixture obtained in step (b). The gas may be introduced by mixing; such as tapping, stirring and whipping; bubbling, or any other suitable method of forming a foam.

Thus, foaming may be performed by mixing a mixture comprising CNF and at least one anionic surfactant in the presence of a gas. Alternatively, foaming may be carried out by blowing a gas into the mixture or adding a foaming agent.

For a given method of introducing gas into the mixture, the gas content of the wet foam may decrease with increasing CNF concentration, as shown in fig. 3 and 4. Meanwhile, the density of the porous solid material may increase when the CNF concentration increases and/or less gas is introduced into the mixture.

The wet foam obtained in process (c) is stable for a period of time long enough to dry without collapsing and largely maintain the fine cell structure of the wet foam. One advantage is that the wet foam obtained in step (c) of the process of the present invention can be transferred to another location, such as a solid surface or a mold, before it is dried, while substantially maintaining the porous structure of the wet foam.

Porosity of porous solid material phi Using equation [2]Calculation where p is the density of the porous solid material according to the invention and p_{Cellulose, process for producing the same, and process for producing the same}Is the density of the dry solid cellulose.

The drying of the wet foam in step (d) of the process of the invention may be carried out at 5-120 ℃, 5-95 ℃, 5-80 ℃, 10-70 ℃, 10-60 ℃, 10-50 ℃, 20-50 ℃ or 35-45 ℃; or by subjecting the wet foam to a temperature of 5-120 deg.C, 5-95 deg.C, 5-80 deg.C, 10-70 deg.C, 10-60 deg.C, 10-50 deg.C, 20-50 deg.C or 35-45 deg.C; until it reaches a liquid content of less than 98 wt%, or less than 90 wt%, less than 80 wt%, less than 70 wt%, less than 60 wt%, or even less than 50 wt% of the total weight of the wet foam, and then raising the temperature to a temperature above the boiling point of the aqueous solvent used to disperse the CNF in step (a). The liquid content of the porous solid material after drying may be 0 wt%, or at least 5 wt%, at least 10 wt%, at least 20 wt%, at least 30 wt%, or at least 40 wt%. The drying of the foam in step (d) may be carried out at a pressure of from 5 to 1000kPa, from 10 to 500kPa, from 20 to 400kPa, from 30 to 300kPa, from 40 to 200kPa or preferably from 50 to 150 kPa. Thus, resource intensive methods for drying wet foams, such as supercritical extraction and freeze drying, can be avoided. Drying at the temperature and pressure according to the invention has the following advantages: porous solid materials are not prone to cracking, particularly when formed into large structures and sheets. It is also possible to maintain the porous structure when the foam is dry.

Drying of the wet foam may result in a reduction in volume. The reduction is mainly due to the volume of moisture evaporating from the foam.

The method may further comprise the step of sheeting the wet foam of step (c) prior to drying. Preferably, prior to drying, the foam is formed into a sheet having a thickness of at least 0.1mm, or at least 0.5mm, at least 1mm, at least 5mm, at least 10mm, or at least 20 mm. The shaped sheet may have a thickness after drying of at least 0.05mm, or at least 0.1mm, or at least 0.2mm, or at least 0.5mm, or at least 1mm, or at least 2mm, or at least 5mm, or at least 10 mm. The options available for drying and removing liquid content from the wet foam, such as the size of the production equipment and the time available for producing the porous solid material, affect the maximum material thickness available. Typically, the porous solid material may be provided with a thickness of up to (and including) 500cm, up to (and including) 100cm, or up to (and including) 50 cm.

The porous solid material according to the invention has a density of less than 500kg/m³Less than 300kg/m³Less than 200kg/m³Less than 100kg/m³Or less than 50kg/m³. The density of the cellulosic solid material may be at least 0.5kg/m³Or at least 1.0kg/m³。

In the porous solid material according to the invention, at least 50% of the total pore volume of the material consists of pores having the following diameters: at least 10 μm, at least 50 μm, at least 100 μm, at least 150 μm, at least 200 μm, at least 250 μm, at least 300 μm, at least 400 μm, at least 500 μm, at least 750 μm, or at least 1000 μm.

The resistance to elastic deformation of an object or substance can be characterized by the young's modulus. The young's modulus E in compression is defined as the ratio of the stress (force per unit area) along the axis to the compressive strain (deformation ratio with respect to the initial length) along the axis in the stress range governed by hooke's law, i.e., in the initial linear portion of the compressive stress-strain curve, and is calculated by equation [3 ].

E is Young's modulus; f is the force exerted on the object under compression; a. the₀Is the original cross-sectional area of the applied force; Δ L is the amount by which the length of the object changes; l is₀Is the original length of the object.

For the purposes of the present invention, the young's modulus as given herein is measured in a direction perpendicular to the surface of the porous solid material sheet dried on a plane prior to measurement. The specific modulus used here is calculated as young's modulus divided by the density of the sample.

The porous solid material of the invention may have a specific modulus of at least 1 kPa/(kg/m)³) At least 5 kPa/(kg/m)³) At least 10 kPa/(kg/m)³) At least 15 kPa/(kg/m)³) At least 20 kPa/(kg/m)³) At least 25 kPa/(kg/m)³) Or at least 30 kPa/(kg/m)³). The porous solid material of the present invention may have a specific modulus of up to (and including) 100 kPa/(kg/m)³) Or even up to and including 150 kPa/(kg/m)³)。

The porous solid material according to the invention can be obtained, for example, by adding CaCO to the dispersion₃And gluconolactone. Alternatively, the porous solid material according to the present invention may be obtained without using a crosslinking agent.

The porous solid material according to the invention may further comprise one or more additives, such as additives selected from the group consisting of: plasticizers, such as glycerin, xylitol, sorbitol, maltitol, sucrose, glucose, fructose, xylose, polyethylene glycol, propylene glycol, butylene glycol, glycerin, triacetin, and urea; inorganic or organic nanoparticles, such as silica nanoparticles, and carbon nanotubes; clays, such as sodium-montmorillonite, kaolinite, chlorite, and mica; cellulose nanocrystals; and polymers including, but not limited to, hemicellulose, lignin, lignosulfonates, cellulose derivatives, starch, other biopolymers, and synthetic polymers.

Nevertheless, the porous solid material according to the invention may comprise substantially no fibrous material other than CNF. Furthermore, the wet foam obtained in step (c) of the process according to the invention may contain no fibrous material other than CNF.

Preferably, the porous solid material according to the present invention may comprise solely anionic cellulose nanofibres, at least one anionic surfactant and optionally a gas or an aqueous liquid, and essentially no other components, such as plasticizers, cross-linking agents, inorganic or organic nanoparticles, clays, cellulose nanocrystals, or polymers. Thus, the porous solid material according to the present invention can be obtained without adding additives such as plasticizers, crosslinking agents, inorganic or organic nanoparticles, clays, cellulose nanocrystals, or polymers in its preparation process.

The porous solid material according to the invention can absorb liquids without losing its porous structure.

Another aspect of the invention is the use of the porous solid material according to the invention as at least one material selected from the group consisting of: an insulating material; a packaging material; absorbent materials and building materials. Examples of insulating materials which can be used with the porous solid material according to the invention are building insulation, sound insulation, thermal insulation and electrical insulation. Examples of absorbent materials are liquid-absorbent, for example for diapers and wound dressings; energy absorption (e.g., shock absorption); and a sound absorbing material. Examples of uses as building materials are lightweight structural components in building structures, sandwich panels, flotation devices and furniture, for example as upholstery and gaskets.

Another aspect of the invention is a product comprising a porous solid material according to the invention. Examples of such products include slabs; a sheet material; layers, such as in a laminate; and a molded structure.

Slabs, sheets and layers formed from the porous solid material according to the invention may have a thickness after drying of at least 0.05mm, or at least 0.1mm, or at least 0.2mm, or at least 0.5mm, or at least 1mm, or at least 2mm, or at least 5mm, or at least 10 mm. The slabs, sheets and layers may have a thickness of up to (and including) 500cm, up to (and including) 100cm, or up to (and including) 50 cm.

Examples

The porous solid materials according to the invention and comparative examples are illustrated in the following examples.

Material

Cellulose Nanofibers (CNF) porous solid materials were prepared using different grades of Cellulose Nanofibers (CNF). The different grades are described below.

Similar to the previously described method (Henriksson et al, 2007), an enzyme-modified CNF (Enz-CNF) is prepared from pulp fibers that are subjected to enzymatic pre-treatment and then defibrinated (defibrinated) in a high pressure homogenizer.

TEMPO-CNF from a commercial sulfite softwood dissolving pulp having a hemicellulose content of 4.5% and a lignin content of 0.6% (II)

Dissolving Pulp；

Fabriker AB，

Sweden). The never-dried dissolving pulp fibers were oxidized using TEMPO according to the method described previously (Saito et al 2007). The fibers were suspended in water containing TEMPO and NaBr. TEMPO-mediated oxidation of cellulose fibers was initiated by addition of NaClO, and pH10 was maintained by continuous addition of NaOH. When the pH10 was maintained without NaOH consumption, the pH was adjusted to pH 5 with HCl. The TEMPO oxidized fiber was then filtered and washed rigorously with deionized water. The TEMPO oxidized fibres were diluted to the desired concentration, typically 1%, and defibrated in a home blender (Magrini, Vita-Prep 3, 1200W) until a viscous dispersion of CNF was formed. The charge of TEMPO-CNF can be controlled by adding varying amounts of NaClO. TEMPO-CNF charge density as measured by polyelectrolyte titration varied between 284-1472. mu. eq/g (

L., et al, nord. pulp pap. res.j.1989, 4, 71-76).

The carboxymethylation pretreatment of the fibers to prepare the carboxymethylated NFC was carried out by InnventiaAB company, stockholm, sweden, with the aid of a high-pressure homogenization technique. First, never dried fiber was dispersed in deionized water at 10000 revolutions using a common laboratory rinser. The fibers were then washed four times with ethanol and intermediate filtered to change the solvent of the fibers to ethanol, and the fibers were impregnated for 30 minutes with a solution of 10g monochloroacetic acid in 500mL isopropanol. These fibers were added in proportion to a solution of NaOH, methanol and isopropanol heated just below the boiling pointThe carboxymethylation reaction was allowed to continue for one hour. After the carboxymethylation step is completed, the fibers are sequentially filtered and cleaned through three steps: deionized water was used first, then acetic acid (0.1M), and finally again. Then, NaHCO was used₃The fiber was impregnated with the solution (4 wt% solution) for 60 minutes, thereby converting the carboxyl group to its sodium salt form. Finally, the fibers were washed with deionized water and drained using a buchner funnel. After the above treatment was completed, the fibers were passed through a high pressure homogenizer (Microfluidizer M-110EH, Microfluidics Corp). The homogenizer had two series-connected chambers (200 μm and 100 μm) of different sizes. Homogenization was achieved by a single feed at a fiber consistency of 2 wt% in aqueous solution, with a fiber charge density of 647 μ eq/g as determined by conductometric titration (Katz s., et al, svensk papperserting, 1984, R87).

2,2,6, 6-tetramethylpiperidine-1-oxide (TEMPO) was purchased from Fluorochem Ltd (Hadfield, UK). Sodium hypochlorite (NaClO) was purchased from Applichem GmbH (Darmstadt, Germany). NaBr and NaOH were purchased from Sigma-Aldrich (Munich, Germany) and HCl was purchased from Th.Geyer GmbH (Renningen, Germany). Glycerol and Sodium Dodecyl Sulfate (SDS) were purchased from Applichem GmbH (Darmstadt, Germany). Glyceryl triacetate was purchased from Sigma-Aldrich (Munich, Germany). Sorbitol and 1-octylamine were purchased from Alfa Aesar GmbH (Karlsruhe, Germany). Potassium oleate was purchased from Sigma-Aldrich (Munich, Germany). Sodium oleate was purchased from Carl Roth GmbH (Karlsruhe, Germany). Sodium Lauryl Ether Sulfate (SLES) was purchased from th.geyer (Segeltorp, Sweden).

Hydroxyethyl cellulose with a molecular weight of 90kDa was purchased from Aldrich. Sodium montmorillonite (Cloisitena)⁺Cation exchange capacity 92mequiv/100g) was obtained from Andrea Jennow A/S (

Denmark). Sodium montmorillonite (MTM) was swollen in water for 24h and then dispersed by vigorous stirring before use.

Characterization of

Wet foam stability. Wet foam stability was evaluated as the foam volume V at time t divided by t ═ tOriginal foam volume at 0V₀。

Drop/bubble distribution tension measurement (DPT) is a known technique for determining the mechanical properties of a liquid-gas interface. This technique is based on the fact that the geometry of the pendant drop can be related to the surface tension of the liquid under study. Gravity attempts to stretch the drop, while surface tension attempts to maintain the spherical surface of the drop. In a DPT apparatus, a drop of liquid is suspended from the end of a tube by surface tension, and the shape of the drop is continuously monitored by a computer, which records the form as a function of the change in area, since the volume of the drop oscillates with a sinusoidal variation. When surfactants are present in the liquid, they are capable of adsorbing and desorbing at the liquid-gas interface during shaking. Depending on the frequency of the oscillations and the adsorption kinetics of the surfactant, the surface tension will vary in a sinusoidal manner, which is out of phase with the oscillations of the surface area. A fast adsorbing surfactant measured at low frequencies will show only a relatively small amplitude in the surface tension change. However, when large molecules or small particles are adsorbed at the interface, they do not have a tendency to desorb from the interface due to the high adsorption energy/particle, and in this case the shape of the droplet will reflect the rheological properties of the liquid-gas interface.

According to equation [4 ]]The complex elastic properties determined with this procedure can be described. From this relationship it is clear that the modulus E at the liquid-gas interface is due to the elastic part (E)₀) And a viscoelastic moiety (2 π ν η), and thus will be frequency dependent, and the frequency of the oscillating droplet should not be higher than the requirements of interfacial mechanical equilibrium. The pendant drop experiment was performed by shaking the drop 10 times under ambient conditions and repeating the measurement every 10 minutes over 1 hour. All experiments were performed with a CNF concentration of 1 g/L.

Where γ is the interfacial tension, A is the area at a given time, A₀Is the area at time 0, Δ γ (t) - γ⁰，ΔA/A₀＝(A(t)-A₀)/A₀，E₀The swelling surface elasticity, v ═ the perturbation frequency and η ═ the swelling surface viscosity.

The increase in the elastic modulus of the liquid-gas interface indicates that the CNF particles adsorb at the liquid-gas interface, and the high elastic modulus is considered advantageous for the preparation of stable wet foams.

Press test the prepared porous solid material was cut into test pieces 1cm square, with a height of 0.4 to 1.5 cm. Compression testing was performed using an Instron 5566 universal tester in an air conditioned room at 23 ℃ and 50% relative humidity. The samples were conditioned for 24 hours at 23 ℃ and 50% relative humidity and then tested according to ISO 844:2007 (E). Using a 500N load cell, the compression rate was 10% of the original sample thickness per minute. The final strain was chosen to be 80% of the original sample height in order to be able to evaluate the material behaviour over a large deformation interval. For all samples, the energy absorbed by the foam was taken as the area under the stress-strain curve between 0% and 80% strain. The specific modulus is calculated as Young's modulus E under compression, divided by the density of the sample, etc., using equation [5 ].

E is the Young's modulus under compression; ρ is the density (in mass/volume) of a specimen of porous solid material, F is the force exerted on the specimen under tension; a. the₀Is the original cross-sectional area of the applied force; Δ L is the amount by which the specimen height changes; l is₀Is the original height of the test piece.

Young's modulus was measured perpendicular to the dry direction.

Porous solid Density and porosity the porosity of the porous solid Material phi is according to equation [2 ]]Calculation where ρ is the density of the porous solid material and ρ_{Cellulose, process for producing the same, and process for producing the same}(1.57g/cm³) Is the density of the dry solid cellulose.

The porous structure of the porous solid material was evaluated by Scanning Electron Microscopy (SEM) using a TM-1000TableTop SEM (Hitachi, Tokyo, Japan). The pore size was measured manually in the SEM image.

Comparative example

Wet foam stability. TEMPO-oxidized anionic CNF (0.1 wt%) was foamed with octylamine and SDS, respectively. The amount of octylamine and SDS added corresponded to 1/3 of the total CNF addition. The foam is generated by hand shaking, discharging the foam at the top of the water column.

Wet foam stability was evaluated as the foam volume V at time t divided by the original foam volume V at time t 0₀See fig. 1. When octylamine was used, the wet foam was relatively stable over a long period of time after an initial small reduction in foam volume, see fig. 1. This indicates that octylamine lyophobic and surface-active CNF and that the foam is particle-stable, with the modified CNF adsorbed at the gas/liquid interface. With SDS, collapse was rapid and after less than one hour, the wet foam completely collapsed. This indicates that SDS does not adsorb to CNF and does not render CNF surface active. From this experiment, it seems unlikely that SDS could be used to form a particle-stable foam to the extent that CNF is surface active and adsorbs to the gas/liquid interface.

Figure 2 shows complex elastic modulus determined by drop/bubble distribution tensiometry, corresponding to CNF (diamonds) and CNF in admixture with octylamine (squares), SDS (circles) without added surfactant, respectively. The filled symbols indicate that the amount of surfactant (mol) corresponds to 1/3 of the total amount of CNF added. Open symbols indicate that the amount of surfactant (mol) corresponds to 1/1 of the total CNF addition. Figure 2 shows that the modulus of elasticity of the mixture containing CNF and octylamine is much higher than that of the mixture containing CNF and SDS. SDS does not significantly change the elastic modulus compared to CNF without added surfactant. This indicates that SDS does not adsorb to CNF, and that CNF is also not surface active, and it should not be possible to prepare stable CNF foams using SDS.

The characteristics of the porous solid material according to the invention are further illustrated in the following examples.

Examples 1-7 illustrate that different surfactants, CNF starting materials and preparation conditions can be used to prepare porous solid materials comprising CNF and anionic surfactant.

Examples 8-14 illustrate the range of properties of porous solid CNF materials that can be obtained by certain specific combinations of CNF, surfactants and additives. According to the literature, wet foams of these anionic CNFs in combination with different anionic surfactants are not expected to be sufficiently stable to dry and maintain the porous structure, supported by wet foam stability and pendant drop measurements. However, the following examples show that they are sufficiently stable to dry and maintain a porous structure.

Example 15 illustrates porous solid materials dried at different temperatures.

Example 1

Different types of surfactants were used to prepare porous solid CNF materials with retained porous structure. 30g of TEMPO-oxidized anionic CNF dispersion (0.5% by weight, surface charge 983. mu. eq/g) were combined with different anionic surfactants, see Table 1. The different combinations were mixed, adjusted to pH8 and foamed for 5-10 minutes using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3 cm). The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. Table 1 shows that a variety of anionic surfactants can be used to prepare a porous solid CNF material with a retained porous structure. These examples were not optimized with respect to the amount of surfactant, density, pore size, thickness or mechanical properties.

Table 1. porous solid CNF materials containing different surfactants.

Calculated based on the total weight of the porous solid material

Example 2

Air content and density of the porous solid material. To 30g of TEMPO oxidized anionic CNF dispersion (surface charge 983 μ eq/g) at different CNF concentrations (0.1 wt% to 1.0 wt%) was added SDS solution (20g/L) and the combined fluids were mixed, adjusted to pH8 and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter ═ 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃.

In the first set of experiments, the SDS loading was kept constant at 0.1ml SDS solution (20g/L) in 30g CNF dispersion. Figure 3 shows the air content (solid diamonds) and the density of the porous solid material (open diamonds) in these wet foams as a function of the CNF dispersion concentration during foaming. For the lowest CNF concentrations (0.1 wt% and 0.2 wt%), the air content in the wet foam was high. However, these foams are not stable enough to collapse upon drying. The air content decreased with increasing CNF concentration, probably due to the increased viscosity of CNF at higher concentrations, which, along with the mixing devices used in these experiments, made the introduction of air more difficult with increasing CNF concentration. As the air content in the wet foam decreases, the density of the dry porous solid material increases with increasing CNF concentration. At a1 wt% CNF concentration, the density at the highest concentration was very high, peaking at almost 800kg/m³. For a given mixing device and a given SDS loading per volume of CNF dispersion, the air content in the wet foam decreases and the density of the dry porous solid material increases with increasing CNF concentration during foaming.

In the second set of experiments, the SDS loading was kept constant at 20mg SDS per gram dry CNF, i.e. the absolute SDS loading was higher the CNF concentration. Figure 4 shows the air content in the wet foam (solid diamonds) and the density of the dry porous solid material (open diamonds) as a function of the CNF dispersion concentration during foaming. For the lowest CNF concentration (0.1 wt% CNF), the foam was very unstable, it had collapsed even before the air content could be measured, and therefore a porous solid material could not be obtained. At CNF concentrations of 0.2 wt% to 1 wt%, the trend is the same as in fig. 3, with increasing CNF concentration during foaming, the air content in the wet foam decreases and the density of the dry porous solid material increases. However, since the SDS loading at higher CNF concentrations was higher than in FIG. 3,the reduction in air content and the increase in density are not as significant as in fig. 4. At a CNF concentration of 1 wt%, the air content in the wet foam was still 14 wt% and the density of the dry porous solid material was 74kg/m³In contrast, the 1 wt% CNF concentration in FIG. 3 is 2 wt% air and 791kg/m³。

Example 3

30g of CNF dispersions of different grades and charge densities were combined with SDS (20g/L), see Table 2. The different combinations were mixed, adjusted to pH8 and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter ═ 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃.

Table 2 shows that a variety of CNFs with different properties can be used to prepare porous solid CNF materials.

TABLE 2

Example 4

30g of TEMPO-CNF dispersion (0.97% by weight, surface charge 983. mu. eq/g) were combined with 0.26ml SDS (25g/L), mixed and adjusted to pH 4.7 and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 88kg/m³。

Example 5

30g of TEMPO-CNF dispersion (0.97% by weight, surface charge 983. mu. eq/g) were combined with 0.26ml SDS (25g/L), mixed and adjusted to pH 6.25 and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 85kg/m³。

Example 6

40g of the Enz-CNF dispersion (3 wt%) was combined with potassium oleate (0.6mL, 25g/L), mixed, adjusted to pH 9, and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 275kg/m³Corresponding to a porosity of 82.5%. The dry porous solid material has a porous structure in which the pores originate from gas bubbles in the wet foam.

Example 7

The Enz-CNF dispersion (3 wt%) was combined with different proportions of TEMPO-CNF dispersion and optionally deionized water was added, see table 3. To each combination was added 0.4mL of sodium oleate (25 g/L). The different combinations were mixed, adjusted to pH 9 and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter ═ 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The dry foam has a porous structure in which the pores originate from the air bubbles in the wet foam. Table 3 shows the density ranges of the porous solid materials obtained by combining Enz-CNF and TEMPO-CNF in different proportions.

TABLE 3

Example 8

30g TEMPO-oxidized anionic CNF dispersion (0.6 wt%, surface charge 983. mu. eq/g) and 0.1ml anionic SDS (25g/L) were mixed, adjusted to pH8 and foamed using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3cm) for 5 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 9.3kg/m³Corresponding to a porosity of 99.4%. An SEM image of the porous solid material can be seen in fig. 5. The porous solid material has a porous structure in which the pores originate from gas bubbles in the wet foam. The size of the pores is typically in the range of a few hundred μm, with most pores being smaller than 500 μm. When the porous solid material is compressedYoung's modulus of 138kPa, and specific modulus of 14.8 kPa/(kg/m)³) And the energy absorption at 70% compression is 33kJ/m³. Even if it is compressed by 80%, the porous solid material does not collapse completely, but after the compressive load is removed it regains a significant portion of its original height.

Example 9

300g of TEMPO-oxidized anionic CNF dispersion (0.5% by weight, surface charge 983. mu. eq/g) and 1.0ml of anionic SDS (25g/L) were mixed, adjusted to pH8 and foamed for 5 minutes using a laboratory mechanical stirrer at 2000rpm and a large impeller (diameter 4.5 cm). The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 7.5kg/m³Corresponding to a porosity of 99.5%. An SEM image of the porous solid material can be seen in fig. 6. The porous solid material has a pore structure in which the pores originate from gas bubbles in the wet foam. The size of the cells is generally larger than the foam in example 4, with most of the cells being larger than 500 μm and approaching 1 mm. Mixing was performed with a larger impeller than in example 4, which may affect the size of the bubbles. Thus, the pore size of the dry porous solid material can be tailored by selecting different impellers or by otherwise introducing air into the wet foam. The porous solid material had a Young's modulus under compression of 281kPa, which gave a specific modulus of 37.5 kPa/(kg/m)³) And an energy absorption at 70% compression of 18kJ/m³. When compressed, the foam collapses and does not regain any of its original height after the compressive load is removed. This different behavior from example 4 may be partly due to the difference in pore size, and this emphasizes that the properties of the porous solid material can be tailored by using different foaming techniques.

Example 10

45g of TEMPO-oxidized anionic CNF dispersion (0.6% by weight, surface charge 983. mu. eq/g) and 1.5ml of anionic SDS (25g/L) were mixed, adjusted to pH8 and foamed for 5 minutes using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3 cm). The resulting wet foam was poured into a plastic tray,and dried in a fan oven at 40 c. The density of the dried porous solid material was very low, 4.4kg/m³Corresponding to a porosity of 99.7%. The Young's modulus under compression was 52kPa, and the specific modulus obtained was 11.8 kPa/(kg/m)³) And an energy absorption at 70% compression of 3kJ/m³。

Example 11

30g TEMPO-oxidized anionic CNF dispersion (0.6 wt%, surface charge 983. mu. eq/g), 0.065g glycerol and 0.1ml anionic SDS (25g/L) were mixed, adjusted to pH8 and foamed for 5 minutes using a laboratory mechanical stirrer at 2000rpm and a small impeller (diameter 3 cm). The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid was 13.2kg/m³Corresponding to a porosity of 99.2%. The Young's modulus under compression was 81kPa, and the specific modulus obtained was 6.1 kPa/(kg/m)³) And an energy absorption at 70% compression of 32kJ/m³. The addition of glycerol makes the sample more flexible.

Example 12

20g of TEMPO-CNF dispersion (1.5% by weight, surface charge approximately 1000. mu. eq/g) were combined with 0.3ml SDS (25g/L) and mixed with 20ml of montmorillonite clay dispersion (5g/L) and adjusted to pH10 and foamed using a laboratory mechanical stirrer at 1500rpm and a small impeller (diameter 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 70kg/m³。

Example 13

20g of TEMPO-CNF dispersion (1.5% by weight, surface charge approximately 1000. mu. eq/g) were combined with 0.2ml SDS (25g/L) and mixed with 20ml of hydroxyethylcellulose solution (5g/L) and adjusted to pH10 and foamed using a laboratory mechanical stirrer at 1500rpm and a small impeller (diameter 3cm) for 5-10 minutes. The resulting wet foam was poured into a plastic tray and dried in a fan oven at 40 ℃. The density of the dried porous solid material was 80kg/m³。

Example 14

350g of TEMPO-CNF dispersion (0.54% by weight, surface charge about 1000. mu. eq/g) were combined with potassium oleate and 1.25g (corresponding to 40% of dry porous solid material) of glycerol, triacetin and sorbitol, respectively. The mixture was adjusted to pH10 and foamed using a laboratory mechanical stirrer at 1500rpm and large impeller (diameter 7cm, height 5cm) for 5-10 minutes. The resulting wet foam was poured into a wooden frame mounted on a PTFE coated baking plate and dried in a fan oven at 80 ℃.

Table 4 shows a summary of the solid porous materials prepared with different plasticizers. The dry porous solid material has a porous structure in which the pores originate from gas bubbles in the wet foam.

TABLE 4

Example 15

3 foams were prepared and dried at different temperatures:

foam 1: 350g of TEMPO-CNF dispersion (0.54% by weight, surface charge approximately 1000. mu. eq/g) were mixed with potassium oleate (1mL, 25g/L) and 0.47g of glycerol, adjusted to pH10 and foamed using a laboratory mechanical stirrer at 1500rpm and large impeller (diameter 7cm, height 5cm) for 5-10 minutes. The resulting wet foam was poured into a wooden frame on a PTFE coated baking plate and dried in a fan oven at 80 ℃.

Foam 2: 300g of TEMPO-CNF dispersion (0.75 wt%, surface charge approximately 1000. mu. eq/g) was mixed with potassium oleate (2mL, 25g/L) and 2.25g of glycerol, adjusted to pH10, and foamed using a laboratory mechanical stirrer at 1500rpm and large impeller (diameter 7cm, height 5cm) for 5-10 minutes. The resulting wet foam was poured into a wooden frame on a PTFE coated baking plate and dried in a fan oven at 90 ℃.

Foam 3: 600g of TEMPO-CNF dispersion (0.5% by weight, surface charge approximately 1000. mu. eq/g) were mixed with SDS (4.4mL, 25g/L) and 1.29g of glycerol, adjusted to pH10 and foamed using a laboratory mechanical stirrer at 1500rpm and large impeller (diameter 7cm, height 5cm) for 5-10 minutes. The resulting wet foam was poured into a wooden frame on a PTFE coated baking plate and dried in a fan oven at 120 ℃.

Table 5 shows a summary of solid porous materials dried at high temperature. The dry porous solid material has a porous structure in which the pores originate from gas bubbles in the wet foam.

TABLE 5

Claims

1. A porous solid material comprising at least 50 wt% Cellulose Nanofibres (CNF) and at least 0.2 wt% but less than 5 wt% of surfactant, calculated on the total weight of the porous solid material, wherein:

a) the surfactant is an anionic surfactant;

b) said material having a density of less than 100kg/m³(ii) a And is

2. The porous solid material according to claim 1, wherein the cellulose nanofibers are anionic cellulose nanofibers.

3. The porous solid material of claim 1, wherein the anionic surfactant is selected from sodium dodecyl sulfate, Sodium Lauryl Ether Sulfate (SLES), sodium oleate, and potassium oleate, or a combination thereof.

4. The porous solid material according to claim 1, wherein the CNF has a charge density of 0 to 2000 μ eq/g.

5. The porous solid material of claim 1, wherein said CNF is any one of an enzyme-modified CNF, a TEMPO-oxidized CNF, or a carboxy-methylated CNF, or a combination thereof.

6. The porous solid material of claim 1 wherein at least 50% of the pores of the material have a diameter of at least 200 μm.

7. The porous solid material of claim 1, wherein the material has at least 1 kPa/(kg/m)³) The specific modulus of (a).

8. The porous solid material according to claim 1, wherein the material comprises no fibrous material other than CNF.

9. The porous solid material of claim 1, wherein the material does not comprise a cross-linking agent.

10. A sheet formed from the material of claim 1 having a thickness of at least 0.05 mm.

11. Preparing a Cellulose Nanofiber (CNF) comprising at least 50 wt% and at least 0.2 wt% but less than 5 wt% of a surfactant, calculated on the total weight of the porous solid material, and having a density of less than 100kg/m³The method of drying a porous solid material of (1), comprising the steps of:

a) providing a dispersion comprising Cellulose Nanofibers (CNF) in an aqueous solvent,

b) adding an anionic surfactant to the dispersion in (a) to obtain a mixture;

c) preparing a wet foam from the mixture obtained in (b), wherein the wet foam has a density of less than 98% of the mixture prepared in (b); and

d) drying the wet foam obtained in (c) to obtain a porous solid material.

12. The method according to claim 11, wherein the anionic surfactant is selected from Sodium Dodecyl Sulfate (SDS), Sodium Lauryl Ether Sulfate (SLES), sodium oleate, and potassium oleate, or a combination thereof.

13. The process according to claim 11, wherein the concentration of CNF in the dispersion obtained in step a) is at least 0.2 wt% of the total weight of the dispersion.

14. The method according to claim 11, wherein the CNF has a charge density of 0 to 2000 μ eq/g.

15. The method according to claim 11, wherein the CNF is any one of an enzyme-modified CNF, a TEMPO-oxidized CNF or a carboxy-methylated CNF, or a combination thereof.

16. The method according to claim 11, wherein the method further comprises the step of forming the foam into a sheet having a thickness of at least 0.1mm prior to drying.

17. The method according to claim 11, wherein the solvent has a water content of at least 80%.

18. The method according to claim 11, wherein the solvent is water.

19. The process according to claim 11, wherein the preparation of the foam in (c) is carried out by introducing a gas into the mixture obtained in (b).

20. The method according to claim 19, wherein the introduction of the gas is performed by mixing the mixture in the presence of the gas, or by blowing the gas into the mixture.

21. A method according to any one of claims 19 to 20, wherein the gas is air.

22. The method according to claim 11, wherein the drying is carried out at a temperature of 5-120 ℃.

23. The process according to claim 11, wherein the drying is carried out at a pressure of 5 to 1000 kPa.

24. The method according to claim 11, wherein the drying is performed without freeze-drying or supercritical drying.

25. The process according to claim 11, wherein the wet foam obtained in step c) contains no fibrous material other than CNF.

26. The method according to claim 11, wherein at least 50% of the pores in the resulting dry porous solid material have a diameter of at least 10 μm.

27. The process according to claim 11, wherein the resulting dry porous solid material is in the form of a sheet having a thickness of at least 0.05 mm.

28. A porous solid material obtained by the method of claim 11.

29. Use of the porous solid material according to claim 1 in at least one material selected from the group consisting of: insulation, packaging, absorbent and construction materials.